Heretofore, a polyester resin such as polyethylene terephthalate has been used for various applications, e.g. for films, sheets or bottles, since it is excellent in various physical properties such as mechanical strength, transparency, electrical characteristics and thermal properties. Especially, its application to containers for beverage products such as carbonated beverages, fruit juice beverages or various other beverages has been expanding.
Such a polyester resin is produced by using a terephthalic acid component and an ethylene glycol component as the main components, and subjecting these components to an esterification reaction or an ester exchange reaction, followed by melt polycondensation and, if necessary, solid phase polycondensation. At that time, an antimony compound is widely used as a polycondensation catalyst.
However, the antimony compound is likely to precipitate as metallic antimony in the polyester resin and may become foreign matters when the resin is formed into films, etc. Further, when it is used as a container for a beverage, the antimony remaining in the resin is feared to elute into the beverage. Accordingly, it is desired not to use such an antimony compound at all during the production and use another metal compound as the catalyst, or to use it in a very small amount if used.
As the polycondensation catalyst, a germanium compound can be used instead of the antimony compound. However, the germanium compound is expensive since its production amount is small, and it is disadvantageous from the viewpoint of costs to use it for the production of a polyester. Various proposals have been made with respect to a method for using an inexpensive and safe titanium compound as a substitute. A titanium compound as a polycondensation catalyst has high activities as compared with an antimony compound or a germanium compound, and the amount to be used may be small, and it is inexpensive as compared with the germanium compound. Thus, the titanium compound is a catalyst highly worth consideration of its industrial application.
On the other hand, the polyester resin is formed into various molded products such as films sheets or bottles by melt molding. However, during such melt molding a cyclic trimer contained in the resin or a by-product such as a cyclic trimer formed during the melt molding, has been problematic in that, for example, it causes soiling of the casting roll or the like during film forming.
As a method for reducing such a cyclic trimer, etc. during melt molding, a method has been proposed to preliminarily reduce a cyclic trimer, etc. by solid phase polycondensation (Patent Documents 1 and 2). However such a method has been inadequate, since it can not suppress a byproduct such as a cyclic trimer formed during the melt molding.
As a method for suppressing a byproduct such as a cyclic trimer to be formed during the melt molding a method has been proposed wherein after solid phase polycondensation the polycondensation catalyst in the resin is deactivated by hot water treatment (Patent Document 3). However, the effects vary depending upon the type of the polycondensation catalyst, and as a result of the study by the present inventors, it has been found that some effects are observed when a germanium compound is used as the polycondensation catalyst, but no substantial effects are observed when an antimony compound or a titanium compound is used as the polycondensation catalyst.
Further, a method is also proposed wherein to a polyester resin, a different polyester resin containing a phosphorus compound is incorporated to deactivate the polycondensation catalyst (Patent Documents 4 and 5). However, such a method is merely to disclose incorporation of a phosphorus compound, or even if a polyester resin containing a phosphorus compound is disclosed, in order to incorporate the phosphorus compound, the phosphorus compound is blended by an extruder, and by such a method, it has been difficult to produce a polyester resin containing a phosphorus compound. Further, in the examples only cases wherein an antimony compound was used as the polycondensation catalyst, are disclosed and the effects were inadequate and there was a problem such that foreign matters would be formed by precipitation of antimony.
Further, a method of incorporating a polyester resin containing a phosphorus compound to improve the thermal stability of the resulting polyester resin composition, is also disclosed (Patent Document 6). However, such a method is directed only to improvement of the thermal stability of the polyester resin obtained by melt polymerization, and no mention is made with respect to the effects for suppressing formation of an oligomer as a byproduct.
Further, a method of incorporating a polyester resin containing at least 250 ppm, as phosphorus atoms, of a phosphorus compound to control the amount of an oligomer to be formed in the resulting polyester resin composition, is disclosed (Patent Document 7). However, in such a method, an antimony compound in an amount of at least 150 ppm is used as a polymerization catalyst for the polyester resin containing at least 250 ppm, as phosphorus atoms, of a phosphorus compound, and by such a method, it is impossible to prevent a trouble by foreign matters derived from the antimony compound.
Still further, a method has been proposed wherein a polyester resin copolymerized with a phosphorus compound, is employed to deactivate a polycondensation catalyst in a polyester resin formed by using a titanium compound or an aluminum compound as the polycondensation catalyst and simultaneously to suppress the amount of a byproduct cyclic ester oligomer formed during the molding (Patent Documents 8 and 9). However, in this method, no mention is made with respect to a polyester resin having a volume resistivity suitable for film forming, and the method was still inadequate for film forming.
Patent Document 1 JP-A-48-101462
Patent Document 2 JP-A-51-48505
Patent Document 3: JP-A-3-47830
Patent Document 4: JP-A-10-316765
Patent Document 5: J-A-10-251393
Patent Document 6: JP-A-6-170911
Patent Document 7: JP-A-2004-339423
Patent Document 8: JP-A-2005-206747
Patent Document 9: JP-A-2005-213291